Murine IRF8 Mutation Offers New Insight into Osteoclast and Root Resorption.

Interferon regulatory factor 8 (IRF8), a transcription factor expressed in immune cells, functions as a negative regulator of osteoclasts and helps maintain dental and skeletal homeostasis. Previously, we reported that a novel mutation in the gene increases susceptibility to multiple idiopathic cervical root resorption (MICRR), a form of tooth root resorption mediated by increased osteoclast activity. The IRF8 G388S variant in the highly conserved C-terminal motif is predicted to alter the protein structure, likely impairing IRF8 function. To investigate the molecular basis of MICRR and IRF8 function in osteoclastogenesis, we generated knock-in () mice using CRISPR/Cas9 technique modeling the human mutation. The heterozygous (Het) and homozygous (Homo) mice showed no gross morphological defects, and the development of hematopoietic cells was unaffected and similar to wild-type (WT) mice. The Het and Homo mice showed no difference in macrophage gene signatures important for antimicrobial defenses and inflammatory cytokine production. Consistent with the phenotype observed in MICRR patients, Het and Homo mice demonstrated significantly increased osteoclast formation and resorption activity in vivo and in vitro when compared to WT mice. The oral ligature-inserted Het and Homo mice displayed significantly increased root resorption and osteoclast-mediated alveolar bone loss compared to WT mice. The increased osteoclastogenesis noted in mice is due to the inability of mutation to inhibit NFATc1-dependent transcriptional activation and downstream osteoclast specific transcripts, as well as its impact on autophagy-related pathways of osteoclast differentiation. This translational study delineates the IRF8 domain important for osteoclast function and provides novel insights into the mutation associated with MICRR. mutation mainly affects osteoclastogenesis while sparing immune cell development and function. These insights extend beyond oral health and significantly advance our understanding of skeletal disorders mediated by increased osteoclast activity and IRF8's role in osteoclastogenesis.